Surge is a condition occurring within turbomachine compressors wherein the rotating compressor or fan blades experience an aerodynamic stall over at least a portion of the blade surface. Surge may be induced by unstable (turbulent) airflow entering the turbomachine compressor, excessive back pressure, rapid change in angular velocity, etc. Flow instability resulting from a surge condition in a rotor stage in an axial flow turbocompressor may cause surging in adjacent stages, depending upon the severity of the condition.
The occurrence of such compressor surges in a gas turbine engine is familiar to both designers and operators of such engines. Devices for detecting the onset of a surge condition are well known in the art as seen, for example, in U.S. Pat. Nos. 3,868,625 issued to Speigner et al, 4,117,668 issued to Elsaesser et al and 4,594,050 issued to Gaston. Such detectors, monitoring the direction and speed of engine airflow, the rate of change of engine rotor speed, and temperature change in the air inlet, respectively, provide accurate, rapid indication of the onset and continuation of a surge condition.
While the occurrence of a surge condition in an operating gas turbine engine is always of concern to the operator, this concern is most acute in an aircraft propulsion application wherein the occurrence of a surge condition often leads to a reduction in engine thrust and hence a degradation in overall aircraft performance. Of still greater concern is the occurrence of a surge in a single engine aircraft wherein the negative consequences of a loss of thrust are more severe than in a multi-engine aircraft. It should also be noted that single engine aircraft are typically designed to operate at maximum performance levels for a given engine configuration and are often equipped with afterburning thrust augmentors. Such configurations are more prone to the occurrence of a surge condition than "commerical rated" engines wherein larger operational safety margins are provided to reduce the likelihood of the occurrence of a surge condition.
Restoration of a surging engine to normal operation has, in the prior art, typically required completely shutting down the operating engine and restarting after the surge condition has cleared. Such response, while effective, is undesirable for many aircraft applications due to the loss of thrust described above. One recovery system of the prior art is shown in U.S. Pat. No. 4,118,926 issued to Curvino et al wherein a stall detector including a means for restarting the engine following shutdown is presented. The value of the Curvino recovery system is in the automatic restarting of the engine immediately upon verification of the clearance of the surge condition.
Another method for detecting and controlling surge in a motor-driven turbocompressor is disclosed in U.S. Pat. No. 4,594,051 issued to Gaston. The Gaston system includes a temperature sensing surge detection system and an anti-surge control bypass valve for venting the turbocompressor to atmosphere following detection of a surge condition within the compressor. The Gaston system is directed toward a process compressor driven by an independent prime mover and is therefore not directly applicable to an aircraft propulsion application.
What is required is a method and a means for returning a surging gas turbine engine to a normal operating condition without requiring a full shutdown and restart.